In a large bowl, combine shrimp, crab, celery, onions, and bell pepper.
In a small bowl, combine mayonnaise, mustard, Old Bay Seasoning, lemon juice and dill. Mix well to combine. Taste and add additional salt and pepper, if necessary.
Toss salad with dressing until coated.
Serve over salad greens, on a croissant as a sandwich or fill puff pastries for a fun brunch item.